The present invention is directed to the production of metals and their alloys, particularly including refractory metallic alloys such as titanium and zirconium aluminides and amorphous metals.
As the fourth-most plentiful metal in the earth""s crust, titanium is relatively abundant in nature (e.g., as rutile-TiO2 and ilmenite-FeTiO3, and has highly useful properties. However, this refractory metal is unfortunately relatively expensive to extract and reduce from its ores, and difficult to fabricate into useful products in view of its high melting point, sometimes requiring use of film or powder metallurgy techniques such as hot isostatic processing of a powdered or thin film form. It is difficult to purify, and even more expensive to prepare in powder form suitable for advanced powder metallurgical manufacturing processes.
Titanium is conventionally produced by reduction of titanium tetrachloride with magnesium metal in a steel batch retort (the xe2x80x9cKroll processxe2x80x9d). A significant part of the high cost of titanium as a result of the inefficiency and batch nature of the Kroll process which is currently used for its manufacture. This process produces crude titanium xe2x80x9cspongexe2x80x9d which may be intimately contaminated with magnesium chloride and titanium subchlorides, as well as impurities in the magnesium reducing agent. The crude titanium xe2x80x9cspongexe2x80x9d which the Kroll process produces, requires costly vacuum arc refining to produce refined titanium ingots which are suitable for manufacturing use. Subsequent grinding and/or plasma particulation of the refined ingot to produce uniform powders for powder metallurgy and composite manufacture is also relatively expensive.
Titanium forms alloys and intermetallic compounds of significant technical importance. Titanium alloys, and especially titanium aluminides, are important, but costly, materials for aerospace components for propulsion and power. The relatively low density of titanium and titanium alloys, combined with their high specific stiffness, high strength, high corrosion resistance and relative toughness, are particularly desirable in aerospace systems. The efficiency of high-performance propulsion systems and turbines is limited by the high temperature capabilities of materials used for engine components. Relatively lightweight gamma-TiAl based intermetallic alloys have desirable strength to weight and other properties, particularly in comparison with the heavier titanium and nickel-base alloys currently used in combustion and compressor sections of engines. A two-phase (TiAl+Ti3Al), structure distributed as fine or coarse lamellar microstructures including the xcex12 (Ti3Al), orthorhombic (Ti2AlNb) and xcex3 (TiAl) classes of alloys may be particularly optimal for some applications. More sophisticated titanium and TiAl reinforced composite aerospace components, such as advanced SiC-fiber-reinforced titanium alloy aeroengine and structural components, are under development in many countries (including the U.S., France, the U.K. and China). Such advanced composites utilize expensive Ti or TiAl powders and/or foils in their manufacture. [see, e.g., Z. X. Guo, xe2x80x9cTowards Cost Effective Manufacturing Of Ti/SiC Fibre Composites And Componentsxe2x80x9d, Materials Science and Technology, Vol. 14, pp. 864-872 (1998)].
Zirconium and its alloys are of particular use to the nuclear power industry, and chemical and materials industries, and for amorphous metal compositions. The corrosion resistance, mechanical properties and neutron transparency of Zirconium, make Zirconium-based alloys important materials for containing or alloying with uranium fuel, and for the construction of critical components of nuclear reactors. Zirconium also has a wide variety of other uses, as a getter in vacuum tubes, as an alloying agent in steel, in surgical appliances, photoflash bulbs, explosives, fiber spinnerets, and lamp filaments, and as a superconductor (with niobium) to make superconductive magnets. As a refractory metal, Zirconium can be difficult to shape and work. However, a variety of Zirconium-aluminum and similar alloys may be quenched to an amorphous, ductile state. For example, see U.S. Pat. No. 5,980,652, describing amorphous Zrxe2x80x94Al alloys which have significant malleability in their amorphous form. Such amorphous Zirconium alloys typically include aluminum, together with metals such as Fe, Co, Ni or Cu which promote amorphous phase formation. Bulk glass-forming metals based on Ti, Al, Zir and/or Fe which can retain their amorphous state without extremely fast cooling rates typically have three to five or more metallic components with a large atomic-size mismatch to facilitate a high packing density without crystallization. They generally form liquid melts with a small free volume and high viscosity which are energetically close to the crystalline state, because of their high packing density and short-range order, which results in slower ecrystallization kinetics and improved glass forming ability [R. Busch, xe2x80x9cThe Thermophysical Properties of Bulk Metallic Glass-Forming Liquidsxe2x80x9d, JOM, 52 (7) (2000), pp. 39-42. A wide variety of Ti, Al, Zr, and Fe-based glass-forming alloys, such as Laxe2x80x94Alxe2x80x94Ni, Zrxe2x80x94Nixe2x80x94Alxe2x80x94Cu, and Zrxe2x80x94Tixe2x80x94Cuxe2x80x94Nixe2x80x94Be, exhibit very good bulk glass-forming ability with high thermal stability in the supercooled glass state, and low critical cooling rates [A. Inoue, et al., Mater. Trans. JIM 31 (1991), p. 425; T. Zhang, et al., Mater. Trans. JIM, 32 (1991), p. 1005; A. Inoue et al., Mater. Trans. JIM, 32 (1991), p. 609; A. Peker and W. L. Johnson, Appl. Phys. Lett., 63 (1993), p. 2342; all cited references incorporated hereby reference]; Zr41.2Ti13.8Cu10.0Ni12.5Be22.5 (V1) has a very low critical cooling rate of about 1 K/s, which is 5-6 orders of magnitude lower than some earlier metallic glass-forming systems. The difference in Gibbs free energy between an undercooled metal alloy glass and the corresponding crystallized alloy is the driving force for crystallization. When it is low, as in bulk glass forming alloys, glass-forming ability is high as has been done for alloys such as Zrxe2x80x94Tixe2x80x94Cuxe2x80x94Nixe2x80x94Be, and Cuxe2x80x94Tixe2x80x94Zrxe2x80x94Ni. The Gibbs free energy difference for such xe2x80x9cstablexe2x80x9d glass-forming alloys may be only 2-4 Kilojoules per mole, normalized to the melting temperature of the respective alloy, even when cooled to temperatures as low as ⅓ the crystalline melting temperature of the alloy. The metal glass formers with the lowest critical cooling rates have smaller (e.g., less than 2 kJ/mole) Gibbs Free Energy differences than do the glass formers with higher critical cooling rates. The small driving force for crystallization of such bulk metal glass mixtures results from their small free volume, and their short-range order in the supercooled liquid, because the variety of atoms with different sizes in the mixture permits effective packing in the glassy state.
Amorphous alloys containing zirconium and titanium have excellent intrinsic corrosion resistance and mechanical properties, but unfortunately have been very expensive. Powder preparation for powder metallurgy manufacturing is also very expensive.
Zirconium is not scarce in nature, but is expensive to extract and reduce from its ores, because of its very high reactivity and high melting point. It is also difficult to purify magnesium chloride byproduct, and even more expensive to prepare in powder or alloy form suitable for advanced powder metallurgical manufacturing processes. Uniform alloy formation can also be an expensive processing step. Zirconiun occurs chiefly as a silicate in the mineral zircon (ZrSiO4), and as an oxide in the mineral baddeleyite. Zirconium is produced commercially by reduction of chloride with magnesium (the Kroll Process), as well as other methods. Hafnium is invariably found in Zirconium ores, and the separation of Hf from Zr is difficult. Commercial-grade Zirconium accordingly contains from 1 to 3% Hafnium.
Efforts have been made to directly produce titanium powders by reduction of titanium halides in molten salts, and by ultrahigh temperature plasma treatment of TiCl4, but such approaches have not yet found commercial success. Sodium fluorotitanate, Na2TiF6, dissolved in molten cryolite, can be reduced by metallic aluminum to produce a powder of metallic Ti, but requires addition of NaF in stoichiometric amount during the reaction to preserve the liquid cryolite medium, and produces large quantities of sodium fluoroaluminate byproduct. [3Na2TiF6+4Al+6NaF+3Ti, see J. Besida, et al., xe2x80x9cThe Chemical Basis of a Novel Fluoride Route to Metallic Titaniumxe2x80x9d]. Similarly, the Albany Research Center (formerly the U.S. Bureau of Mines) has investigated the reduction of titanium tetrachloride in molten chloride salts, [S. J. Gerdemann, et. al., xe2x80x9cContinuous Production of Titanium Powderxe2x80x9d, at pp. 49-56 in xe2x80x9cTitanium Extraction and Processingxe2x80x9d, Misra and Kipourous, ed., ISBN 0-87339-380-5 (1996); J. C. White and L. L. Oden, xe2x80x9cContinuous Production of Granular or Powder Ti, Zr, Hf or Other Alloy Powdersxe2x80x9d. U.S. Pat. No. 5,259,862,], but purity, separation, oxidation and other issues may present difficulties. Plasma thermal reduction of titanium chlorides is also a recent approach to producing titanium products, but utilizes heating to extremely high temperatures, and is accordingly very energy intensive.
Accordingly, there is a need for efficient, continuous processes to directly produce metals such as titanium and zirconium alloy powders as commodity products, and it is an object of one aspect of the present invention to provide such processes.
There is also a need to produce powder metallurgy materials for use in manufacturing reinforced intermetallic composite and amorphous metallic products, and it is an object of one aspect of the present disclosure to provide such materials and processes for manufacturing them.